How Much Paint Do You Need?

Buying paint is one of the few home-improvement purchases where getting it wrong in either direction costs you a second trip to the store. Order too little and you run out mid-wall, leaving a lap line that's nearly impossible to hide once the first coat dries. Order too much and you're storing half-empty cans in the garage for a decade. The math to get it right takes about five minutes — here's how to do it.

How to Calculate How Much Paint You Need

Paint is sold by coverage, and most latex paints cover 350–400 sq ft per gallon on a smooth, primed surface. To find out how many gallons you need, you need to know your paintable square footage — which is not the same as your room's floor area.

1. Calculate wall area.Add up the perimeter of the room (all four walls), then multiply by ceiling height. For a 12 × 14 ft room with 8-ft ceilings: (12 + 12 + 14 + 14) × 8 = 416 sq ft.
2. Subtract doors and windows.A standard interior door is roughly 20 sq ft (3 × 6.7 ft). A typical window is 12–15 sq ft. Subtract each one from your total. It's fine to ignore very small windows — the rounding gives you built-in waste.
3. Divide by coverage rate. Divide net square footage by the coverage printed on the paint can (usually 350–400 sq ft/gal). The result is gallons per coat.
4. Multiply by number of coats. Most walls need two coats. Multiply accordingly, then round up to the nearest quart increment.

Worked example — 12 × 14 ft bedroom

Room: 12 × 14 ft, 8 ft ceilings
Wall area: (12+12+14+14) × 8 = 416 sq ft
Subtract: 2 windows (15 sq ft each) + 1 door (20 sq ft) = 50 sq ft
Net paintable: 416 − 50 = 366 sq ft

At 350 sq ft/gal: 366 ÷ 350 = 1.05 gal per coat
Two coats: 1.05 × 2 = 2.1 gal → buy 1 gallon + 1 quart per coat, or 3 quarts + 1 gallon

If you want to skip the arithmetic, you can enter your room dimensions and get gallons, quarts, and a cost estimate in the paint calculator once you have your measurements.

Paint Coverage Rates by Type

Coverage varies significantly by product. Thicker coatings — primers, masonry paint, exterior finishes — spread fewer square feet per gallon than interior wall paint. Always check the label; the table below shows typical ranges.

One Coat or Two?

The number of coats is the variable that most people underestimate. Here's when each scenario applies:

• One coat is enoughonly when you're applying the same color over an existing coat of the same sheen, and the existing paint is in good condition. Touch-ups on a freshly painted wall sometimes qualify.
• Two coats is the standard for most paint jobs. It produces even sheen, hides underlying variations in the old surface, and makes the color true to the swatch.
• Three coats may be required when covering a dark color with a light one (e.g., navy → white), painting over a glossy surface without sanding, or using a low-pigment color like a bright yellow or vivid red. These pigments are notoriously transparent and rarely reach full opacity in two coats.
• Primer + two coatsis required on new drywall. Raw drywall paper is very absorbent — skipping primer leads to uneven sheen (called “flashing”) that's visible even after two coats of paint.

Common Mistakes That Lead to Running Out (or Buying Too Much)

• Using floor area instead of wall area.A 200 sq ft bedroom has roughly 400–500 sq ft of paintable wall surface. If you buy paint based on floor area, you'll run short by half.
• Forgetting the ceiling.If you're painting the ceiling too, add the floor area to your total. Most ceilings are flat white and take one coat of ceiling paint — calculate them separately since coverage rates may differ.
• Not accounting for surface texture. Textured walls (orange peel, knockdown, stucco) have more actual surface area than a smooth wall and absorb more paint. Reduce your coverage estimate by 10–15% for heavily textured surfaces.
• Buying exactly enough. Always add 10% to your final number for touch-ups — scuffs, nicks, and future repairs need matching paint, and manufacturers sometimes adjust formulas between batches. A leftover quart stored in a cool, dry space stays usable for 10 years.
• Confusing the trim order.Walls and trim are almost always different products: flat or eggshell for walls, semi-gloss for trim. Calculate them separately and don't mix the two when estimating.

Running a quick calculation before you go to the store is the single biggest time-saver in any paint project. A room that needs 2.5 gallons looks close enough to 2 that many people under-buy — then discover it mid-second coat, when the store is closed.